Functionalized quinacridone pigments

ABSTRACT

Quinacridone pigments that are surface-functionalized with glycidyl methacrylate, maleic anhydride, or 4-methacryloxyethyl trimellitic anhydride to create a functionalized pigment. The functional groups are then activated to bond hydrophobic polymers, thereby coating the pigment with the hydrophobic polymers. The quinacridone pigments can be used for a variety of applications. They are well-suited for use in electro-optic materials, such as electrophoretic media for use in electrophoretic displays.

PRIOR APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 15/347,995, filed Nov. 10, 2016, which claims priority to U.S.Provisional Patent Application No. 62/253,755, filed Nov. 11, 2015. Allof the aforementioned applications are incorporated herein by referencein their entireties.

BACKGROUND OF INVENTION

This invention relates to colored electrophoretic displays, and morespecifically to electrophoretic displays capable of rendering more thantwo colors using a single layer of electrophoretic material comprising aplurality of colored particles.

The term color as used herein includes black and white. White particlesare often of the light scattering type.

The term gray state is used herein in its conventional meaning in theimaging art to refer to a state intermediate two extreme optical statesof a pixel, and does not necessarily imply a black-white transitionbetween these two extreme states. For example, several of the l Inkpatents and published applications referred to below describeelectrophoretic displays in which the extreme states are white and deepblue, so that an intermediate gray state would actually be pale blue.Indeed, as already mentioned, the change in optical state may not be acolor change at all. The terms black and white may be used hereinafterto refer to the two extreme optical states of a display, and should beunderstood as normally including extreme optical states which are notstrictly black and white, for example the aforementioned white and darkblue states.

The terms bistable and bistability are used herein in their conventionalmeaning in the art to refer to displays comprising display elementshaving first and second display states differing in at least one opticalproperty, and such that after any given element has been driven, bymeans of an addressing pulse of finite duration, to assume either itsfirst or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin U.S. Pat. No. 7,170,670 that some particle-based electrophoreticdisplays capable of gray scale are stable not only in their extremeblack and white states but also in their intermediate gray states, andthe same is true of some other types of electro-optic displays. Thistype of display is properly called multi-stable rather than bistable,although for convenience the term bistable may be used herein to coverboth bistable and multi-stable displays.

The term impulse, when used to refer to driving an electrophoreticdisplay, is used herein to refer to the integral of the applied voltagewith respect to time during the period in which the display is driven.

A particle that absorbs, scatters, or reflects light, either in a broadband or at selected wavelengths, is referred to herein as a colored orpigment particle. Various materials other than pigments (in the strictsense of that term as meaning insoluble colored materials) that absorbor reflect light, such as dyes or photonic crystals, etc., may also beused in the electrophoretic media and displays of the present invention.

Particle-based electrophoretic displays have been the subject of intenseresearch and development for a number of years. In such displays, aplurality of charged particles (sometimes referred to as pigmentparticles) move through a fluid under the influence of an electricfield. Electrophoretic displays can have attributes of good brightnessand contrast, wide viewing angles, state bistability, and low powerconsumption when compared with liquid crystal displays. Nevertheless,problems with the long-term image quality of these displays haveprevented their widespread usage. For example, particles that make upelectrophoretic displays tend to settle, resulting in inadequateservice-life for these displays.

As noted above, electrophoretic media require the presence of a fluid.In most prior art electrophoretic media, this fluid is a liquid, butelectrophoretic media can be produced using gaseous fluids; see, forexample, Kitamura, T., et al., Electrical toner movement for electronicpaper-like display, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y., etal., Toner display using insulative particles charged triboelectrically,IDW Japan, 2001, Paper AMD4-4). See also U.S. Pat. Nos. 7,321,459 and7,236,291. Such gas-based electrophoretic media appear to be susceptibleto the same types of problems due to particle settling as liquid-basedelectrophoretic media, when the media are used in an orientation whichpermits such settling, for example in a sign where the medium isdisposed in a vertical plane. Indeed, particle settling appears to be amore serious problem in gas-based electrophoretic media than inliquid-based ones, since the lower viscosity of gaseous suspendingfluids as compared with liquid ones allows more rapid settling of theelectrophoretic particles.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT) and E Ink Corporationdescribe various technologies used in encapsulated electrophoretic andother electro-optic media. Such encapsulated media comprise numeroussmall capsules, each of which itself comprises an internal phasecontaining electrophoretically-mobile particles in a fluid medium, and acapsule wall surrounding the internal phase. Typically, the capsules arethemselves held within a polymeric binder to form a coherent layerpositioned between two electrodes. The technologies described in thesepatents and applications include:

-   -   (a) Electrophoretic particles, fluids and fluid additives; see        for example U.S. Pat. Nos. 7,002,728 and 7,679,814;    -   (b) Capsules, binders and encapsulation processes; see for        example U.S. Pat. Nos. 6,922,276 and 7,411,719;    -   (c) Films and sub-assemblies containing electro-optic materials;        see for example U.S. Pat. Nos. 6,982,178 and 7,839,564;    -   (d) Backplanes, adhesive layers and other auxiliary layers and        methods used in displays; see for example U.S. Pat. Nos.        7,116,318 and 7,535,624;    -   (e) Color formation and color adjustment; see for example U.S.        Pat. Nos. 6,017,584; 6,664,944; 6,864,875; 7,075,502; 7,167,155;        7,667,684; 7,791,789; 7,956,841; 8,040,594; 8,054,526;        8,098,418; 8,213,076; and 8,363,299; and U.S. Patent        Applications Publication Nos. 2004/0263947; 2007/0109219;        2007/0223079; 2008/0023332; 2008/0043318; 2008/0048970;        2009/0004442; 2009/0225398; 2010/0103502; 2010/0156780;        2011/0164307; 2011/0195629; 2011/0310461; 2012/0008188;        2012/0019898; 2012/0075687; 2012/0081779; 2012/0134009;        2012/0182597; 2012/0212462; 2012/0157269; and 2012/0326957;    -   (f) Methods for driving displays; see for example U.S. Pat. Nos.        5,930,026; 6,445,489; 6,504,524; 6,512,354; 6,531,997;        6,753,999; 6,825,970; 6,900,851; 6,995,550; 7,012,600;        7,023,420; 7,034,783; 7,116,466; 7,119,772; 7,193,625;        7,202,847; 7,259,744; 7,304,787; 7,312,794; 7,327,511;        7,453,445; 7,492,339; 7,528,822; 7,545,358; 7,583,251;        7,602,374; 7,612,760; 7,679,599; 7,688,297; 7,729,039;        7,733,311; 7,733,335; 7,787,169; 7,952,557; 7,956,841;        7,999,787; 8,077,141; 8,125,501; 8,139,050; 8,174,490;        8,289,250; 8,300,006; and 8,314,784; and. U.S. Patent        Applications Publication Nos. 2003/0102858; 2005/0122284;        2005/0179642; 2005/0253777; 2007/0091418; 2007/0103427;        2008/0024429; 2008/0024482; 2008/0136774; 2008/0150888;        2008/0291129; 2009/0174651; 2009/0179923; 2009/0195568;        2009/0322721; 2010/0045592; 2010/0220121; 2010/0220122;        2010/0265561; 2011/0187684; 2011/0193840; 2011/0193841;        2011/0199671; and 2011/0285754 (these patents and applications        may hereinafter be referred to as the MEDEOD (MEthods for        Driving Electro-optic Displays) applications);    -   (g) Applications of displays; see for example U.S. Pat. Nos.        7,312,784 and 8,009,348; and    -   (h) Non-electrophoretic displays, as described in U.S. Pat. Nos.        6,241,921; 6,950,220; 7,420,549 and 8,319,759; and U.S. Patent        Application Publication No. 2012/0293858.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display, inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,U.S. Pat. No. 6,866,760. Accordingly, for purposes of the presentapplication, such polymer-dispersed electrophoretic media are regardedas sub-species of encapsulated electrophoretic media.

A related type of electrophoretic display is a so-called microcellelectrophoretic display. In a microcell electrophoretic display, thecharged particles and the fluid are not encapsulated withinmicrocapsules but instead are retained within a plurality of cavitiesformed within a carrier medium, typically a polymeric film. See, forexample, U.S. Pat. Nos. 6,672,921 and 6,788,449, both assigned to SipixImaging, Inc.

Although electrophoretic media are often opaque (since, for example, inmany electrophoretic media, the particles substantially blocktransmission of visible light through the display) and operate in areflective mode, many electrophoretic displays can be made to operate ina so-called shutter mode in which one display state is substantiallyopaque and one is light-transmissive. See, for example, U.S. Pat. Nos.5,872,552; 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971; and6,184,856. Dielectrophoretic displays, which are similar toelectrophoretic displays but rely upon variations in electric fieldstrength, can operate in a similar mode; see U.S. Pat. No. 4,418,346.Other types of electro-optic displays may also be capable of operatingin shutter mode. Electro-optic media operating in shutter mode can beused in multi-layer structures for full color displays; in suchstructures, at least one layer adjacent the viewing surface of thedisplay operates in shutter mode to expose or conceal a second layermore distant from the viewing surface.

An encapsulated electrophoretic display typically does not suffer fromthe clustering and settling failure mode of traditional electrophoreticdevices and provides further advantages, such as the ability to print orcoat the display on a wide variety of flexible and rigid substrates.(Use of the word printing is intended to include all forms of printingand coating, including, but without limitation: pre-metered coatingssuch as patch die coating, slot or extrusion coating, slide or cascadecoating, curtain coating; roll coating such as knife over roll coating,forward and reverse roll coating; gravure coating; dip coating; spraycoating; meniscus coating; spin coating; brush coating; air knifecoating; silk screen printing processes; electrostatic printingprocesses; thermal printing processes; ink jet printing processes;electrophoretic deposition (See U.S. Pat. No. 7,339,715); and othersimilar techniques.) Thus, the resulting display can be flexible.Further, because the display medium can be printed (using a variety ofmethods), the display itself can be made inexpensively.

The aforementioned U.S. Pat. No. 6,982,178 describes a method ofassembling a solid electro-optic display (including an encapsulatedelectrophoretic display) which is well adapted for mass production.Essentially, this patent describes a so-called front plane laminate(FPL) which comprises, in order, a light-transmissiveelectrically-conductive layer; a layer of a solid electro-optic mediumin electrical contact with the electrically-conductive layer; anadhesive layer; and a release sheet. Typically, the light-transmissiveelectrically-conductive layer will be carried on a light-transmissivesubstrate, which is preferably flexible, in the sense that the substratecan be manually wrapped around a drum (say) 10 inches (254 mm) indiameter without permanent deformation. The term light-transmissive isused in this patent and herein to mean that the layer thus designatedtransmits sufficient light to enable an observer, looking through thatlayer, to observe the change in display states of the electro-opticmedium, which will normally be viewed through theelectrically-conductive layer and adjacent substrate (if present); incases where the electro-optic medium displays a change in reflectivityat non-visible wavelengths, the term light-transmissive should of coursebe interpreted to refer to transmission of the relevant non-visiblewavelengths. The substrate will typically be a polymeric film, and willnormally have a thickness in the range of about 1 to about 25 mil (25 to634 μm), preferably about 2 to about 10 mil (51 to 254 μm). Theelectrically-conductive layer is conveniently a thin metal or metaloxide layer of, for example, aluminum or ITO, or may be a conductivepolymer. Poly(ethylene terephthalate) (PET) films coated with aluminumor ITO are available commercially, for example as aluminized Mylar(Mylar is a Registered Trade Mark) from E.I. du Pont de Nemours &Company, Wilmington DE, and such commercial materials may be used withgood results in the front plane laminate.

Assembly of an electro-optic display using such a front plane laminatemay be effected by removing the release sheet from the front planelaminate and contacting the adhesive layer with the backplane underconditions effective to cause the adhesive layer to adhere to thebackplane, thereby securing the adhesive layer, layer of electro-opticmedium and electrically-conductive layer to the backplane. This processis well-adapted to mass production since the front plane laminate may bemass produced, typically using roll-to-roll coating techniques, and thencut into pieces of any size needed for use with specific backplanes.

U.S. Pat. No. 7,561,324 describes a so-called double release sheet whichis essentially a simplified version of the front plane laminate of theaforementioned U.S. Pat. No. 6,982,178. One form of the double releasesheet comprises a layer of a solid electro-optic medium sandwichedbetween two adhesive layers, one or both of the adhesive layers beingcovered by a release sheet. Another form of the double release sheetcomprises a layer of a solid electro-optic medium sandwiched between tworelease sheets. Both forms of the double release film are intended foruse in a process generally similar to the process for assembling anelectro-optic display from a front plane laminate already described, butinvolving two separate laminations; typically, in a first lamination thedouble release sheet is laminated to a front electrode to form a frontsub-assembly, and then in a second lamination the front sub-assembly islaminated to a backplane to form the final display, although the orderof these two laminations could be reversed if desired.

U.S. Pat. No. 7,839,564 describes a so-called inverted front planelaminate, which is a variant of the front plane laminate described inthe aforementioned U.S. Pat. No. 6,982,178. This inverted front planelaminate comprises, in order, at least one of a light-transmissiveprotective layer and a light-transmissive electrically-conductive layer;an adhesive layer; a layer of a solid electro-optic medium; and arelease sheet. This inverted front plane laminate is used to form anelectro-optic display having a layer of lamination adhesive between theelectro-optic layer and the front electrode or front substrate; asecond, typically thin layer of adhesive may or may not be presentbetween the electro-optic layer and a backplane. Such electro-opticdisplays can combine good resolution with good low temperatureperformance.

As indicated above most simple prior art electrophoretic mediaessentially display only two colors. Such electrophoretic media eitheruse a single type of electrophoretic particle having a first color in acolored fluid having a second, different color (in which case, the firstcolor is displayed when the particles lie adjacent the viewing surfaceof the display and the second color is displayed when the particles arespaced from the viewing surface), or first and second types ofelectrophoretic particles having differing first and second colors in anuncolored fluid (in which case, the first color is displayed when thefirst type of particles lie adjacent the viewing surface of the displayand the second color is displayed when the second type of particles lieadjacent the viewing surface). Typically the two colors are black andwhite. If a full color display is desired, a color filter array may bedeposited over the viewing surface of the monochrome (black and white)display. Displays with color filter arrays rely on area. sharing andcolor blending to create color stimuli. The available display area isshared between three or four primary colors such as red/green/blue (RGB)or red/green/blue/white (RGBW), and the filters can be arranged inone-dimensional (stripe) or two-dimensional (2×2) repeat patterns. Otherchoices of primary colors or more than three primaries are also known inthe art. The three (in the case of RGB displays) or four (in the case ofRGBW displays) sub-pixels are chosen small enough so that at theintended viewing distance they visually blend together to a single pixelwith a uniform color stimulus (‘color blending’). The inherentdisadvantage of area sharing is that the colorants are always present,and colors can only be modulated by switching the corresponding pixelsof the underlying monochrome display to white or black (switching thecorresponding primary colors on or off). For example, in an ideal RGBWdisplay, each of the red, green, blue and white primaries occupy onefourth of the display area (one sub-pixel out of four), with the whitesub-pixel being as bright as the underlying monochrome display white,and each of the colored sub-pixels being no lighter than one third ofthe monochrome display white. The brightness of the white color shown bythe display as a whole cannot be more than one half of the brightness ofthe white sub-pixel (white areas of the display are produced bydisplaying the one white sub-pixel out of each four, plus each coloredsub-pixel in its colored form being equivalent to one third of a whitesub-pixel, so the three colored sub-pixels combined contribute no morethan the one white sub-pixel). The brightness and saturation of colorsis lowered by area-sharing with color pixels switched to black. Areasharing is especially problematic when mixing yellow because it islighter than any other color of equal brightness, and saturated yellowis almost as bright as white. Switching the blue pixels (one fourth ofthe display area) to black makes the yellow too dark.

Multilayer, stacked electrophoretic displays are known in the art; J.Heikenfeld, P. Drzaic, J-S Yeo and T. Koch, Journal of the SID, 19(2),2011, pp. 129-156. In such displays, ambient light passes through imagesin each of the three subtractive primary colors, in precise analogy withconventional color printing. U.S. Pat. No. 6,727,873 describes a stackedelectrophoretic display in which three layers of switchable cells areplaced over a reflective background. Similar displays are known in whichcolored particles are moved laterally (see International Application No.WO 2008/065605) or, using a combination of vertical and lateral motion,sequestered into micropits. In both cases, each layer is provided withelectrodes that serve to concentrate or disperse the colored particleson a pixel-by-pixel basis, so that each of the three layers requires alayer of thin-film transistors (TFT's) (two of the three layers of TFT'smust be substantially transparent) and a light-transmissivecounter-electrode. Such a complex arrangement of electrodes is costly tomanufacture, and in the present state of the art it is difficult toprovide an adequately transparent plane of pixel electrodes, especiallyas the white state of the display must be viewed through several layersof electrodes. Multi-layer displays also suffer from parallax problemsas the thickness of the display stack approaches or exceeds the pixelsize.

U.S. Applications Publication Nos. 2012/0008188 and 2012/0134009describe multicolor electrophoretic displays having a single back planecomprising independently addressable pixel electrodes and a common,light-transmissive front electrode. Between the back plane and the frontelectrode is disposed a plurality of electrophoretic layers. Displaysdescribed in these applications are capable of rendering any of theprimary colors (red, green, blue, cyan, magenta, yellow, white andblack) at any pixel location. However, there are disadvantages to theuse of multiple electrophoretic layers located between a single set ofaddressing electrodes. The electric field experienced by the particlesin a particular layer is lower than would be the case for a singleelectrophoretic layer addressed with the same voltage. In addition,optical losses in an electrophoretic layer closest to the viewingsurface (for example, caused by light scattering or unwanted absorption)may affect the appearance of images formed in underlying electrophoreticlayers.

Attempts have been made to provide full-color electrophoretic displaysusing a single electrophoretic layer. See, for example, U.S. PatentApplication Publication No. 2011/0134506. However, in the current stateof the art such displays typically involve compromises such as slowswitching speeds (as long as several seconds) or high addressingvoltages.

The present invention provides polymer functionalized pigments suitablefor use in full-color electrophoretic displays, among otherapplications.

SUMMARY OF INVENTION

The invention provides surface functionalized pigments and polymercoated pigments including the surface functionalization. The pigments ofthe invention are easily dispersed in non-polar and/or hydrophobicfluids, and may be useful for a variety of applications such as paints,printing, e.g., inkjet printing, color filter fabrication, andincorporation into electro-optic displays, e.g., electrophoreticdisplays, where they may be used as a part of the electrophoreticmedium.

The invention includes colored pigments comprising Formula I:

wherein R₁ is a hydrogen, C₁-C₃ alkyl group, or a halogen, or—CH₂CHR₄CH₂OCOR₃, R₂ is a hydrogen, C₁-C₃ alkyl group, or a halogen, R₃is —C(CH₃)CH₂, or a hydrophobic polymer having a molecular weightbetween 5 kD and 100 kD; and R₄ is —OH or—O[CH₂CH(CH2OCOC(CH₃)CH₂)O]_(x)H, and x is an integer from 1 to 15.Typically the pigment is magenta, red, violet, or pink, however multiplespecies of Formula I can be combined to tune a bulk pigment to thedesired color. In one embodiment, the pigment of Formula I is reactedwith a hydrophobic polymer, such as a polymer comprising a methacrylateor acrylate, such as lauryl acrylate, lauryl methacrylate, 2-ethylhexylacrylate, 2-ethylhexyl methacrylate, hexyl acrylate, hexyl methacrylate,n-octyl acrylate, n-octyl methacrylate, n-octadecyl acrylate, orn-octadecyl methacrylate.

In some embodiment, the functionalized pigment is of Formula II:

wherein R₂, R₄, and x are as described above with respect to Formula I.In one embodiment, the pigment of Formula II is reacted with ahydrophobic polymer, such as a polymer comprising a methacrylate oracrylate, such as lauryl acrylate, lauryl methacrylate, 2-ethylhexylacrylate, 2-ethylhexyl methacrylate, hexyl acrylate, hexyl methacrylate,n-octyl acrylate, n-octyl methacrylate, n-octadecyl acrylate, orn-octadecyl methacrylate.

In some embodiments, the polymer functionalized pigment is of FormulaIII:

wherein R₁ and R₂ are as described above with respect to Formula I, andm and n are independently integers between 10 and 200.

In some embodiments, the polymer functionalized pigment is of FormulaIV:

wherein R₂ is as defined above with respect to Formula I, and m and nare independently integers between 10 and 200.

The hydrophobic polymer improves the dispersion of the pigment,especially when it is used in an electrophoretic medium. As a result,electrophoretic media including the pigments of the invention have agreater dynamic range between “white” and “colored” states. The pigmentsof the invention also switch between “white” and “colored” states fasterthan similarly-colored state-of-the-art pigments when both pigments aredriven with the same voltage.

In other aspects, the invention includes colored pigments comprisingFormula VI:

wherein R₁ is a hydrogen, a C₁-C₃ alkyl group, a halogen, a hydroxyl, or—COCR₃CR₄COOH, R₂ is a hydrogen, C₁-C₃ alkyl group, or a halogen, R₃ ishydrogen or a hydrophobic polymer having a molecular weight between 5 kDand 100 kD, and is hydrogen or a hydrophobic polymer having a molecularweight between 5 kD and 100 kD. Typically the pigment is magenta, red,violet, or pink, however multiple species of Formula VI can be combinedto tune a bulk pigment to the desired color. In one embodiment, thepigment of Formula VI is reacted with a hydrophobic polymer, such as amethacrylate or acrylate, such as lauryl acrylate, lauryl methacrylate,2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, hexyl acrylate, hexylmethacrylate, n-octyl acrylate, n-octyl methacrylate, n-octadecylacrylate, or n-octadecyl methacrylate.

In some embodiment, the polymer functionalized pigment is of FormulaVII:

wherein R₁ and R₂ are as defined above with respect to Formula VI, and mand n are independently integers between 10 and 200.

In some embodiment, the polymer functionalized pigment is of FormulaVIII:

wherein R₂ is as defined above with respect to Formula VI, and m and nare independently integers between 10 and 200.

The hydrophobic polymer improves the dispersion of the pigment,especially when it is used in an electrophoretic medium. As a result,electrophoretic media including the pigments of the invention have agreater dynamic range between “white” and “colored” states. The pigmentsof the invention also switch between “white” and “colored” states fasterthan similarly-colored state-of-the-art pigments when both pigments aredriven with the same voltage.

The invention additional includes methods of making functionalizedpigments of Formula V:

wherein R is a hydrogen, C₁-C₃ alkyl group, or a halogen. For example,pigments of Formulas I-IV, and VI-VIII (above) can be created using themethods of the invention. The method includes providing a pigmentcomprising a colored species of Formula V and reacting the pigment withglycidyl methacrylate, maleic anhydride, or 4-methacryloxyethyltrimellitic anhydride monomers to create a functionalized pigment.Functionalizing pigments of Formula V with glycidyl methacrylate, maleicanhydride, or 4-methacryloxyethyl trimellitic anhydride monomers can beachieved faster and with greater efficiency than prior art methods offunctionalizing pigments of Formula V. Once the functionalized pigmentshave been prepared, the functionalized pigments can be combined withhydrophobic polymers, such as a methacrylate or acrylate, such as laurylacrylate, lauryl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexylmethacrylate, hexyl acrylate, hexyl methacrylate, n-octyl acrylate,n-octyl methacrylate, n-octadecyl acrylate, or n-octadecyl methacrylate.In some instances, the hydrophobic polymer is lauryl methacrylate. Insome instances, the hydrophobic polymer and the functionalized pigmentare ball milled prior to reacting.

The described pigments can be used in an electrophoretic mediumcomprising a fluid and first, second, and third species of particlesdisposed in the fluid. The first species of particles bear charges ofone polarity, while the second and third species of particles bearcharges of the opposite polarity. The characteristics of the first,second and third species of particles are such that theparticle-particle interactions are less between the particles of thefirst species and the particles of the second species than between theparticles of the first species and the particles of the third species.When a first addressing impulse is applied to the electrophoreticmedium, the first and third species of particles move in one directionrelative to the electric field and the second species of particles movein the opposed direction relative to the electric field. When a secondaddressing impulse, larger than the first addressing impulse but of thesame polarity is applied to the electrophoretic medium, the firstspecies of particles move in said one direction relative to the electricfield, while the second and third species of particles move in saidopposed direction relative to the electric field.

In another aspect, this invention provides an electrophoretic displaycapable of rendering multiple different colors, the display comprisingan electrophoretic medium and first and second electrodes disposed onopposed sides of the electrophoretic medium. The electrophoretic mediumcomprises a fluid and a plurality of a first species of particles havinga negative charge, a plurality a second species of particles having apositive charge, and a plurality of a third species of particles havinga positive charge.

In another aspect, this invention provides an electrophoretic mediumcomprising a fluid and first, second and third species of particlesdisposed in the fluid. The fluid is dyed a first color. The firstspecies of particles are light-scattering, and bear charges of onepolarity, while the second and third species of particles are non-lightscattering, are of second and third colors respectively different fromthe first color and from each other, and bear charges of the oppositepolarity. The characteristics of the first, second and third species ofparticles are such that the particle-particle interactions are lessbetween the particles of the first species and the particles of thesecond species than between the particles of the first species and theparticles of the third species.

This invention also provides an electrophoretic display capable ofrendering multiple different colors, the display comprising anelectrophoretic medium and first and second electrodes disposed onopposed sides of the electrophoretic medium. The electrophoretic mediumcomprises a fluid dyed a first color; a plurality of a first species oflight-scattering particles having a negative charge; a plurality of asecond species of non-light scattering particles having a second colorand a positive charge; and a plurality of a third species ofnon-light-scattering particles having a third color and a positivecharge.

Finally, the present invention provides an electrophoretic mediumcomprising a fluid and at least one type of charged particle disposed inthe fluid and capable of moving through the fluid when an electric fieldis applied to the medium, the medium further comprising a charge-controladjuvant capable of imparting a more positive charge to the chargedparticles, wherein the charge-control adjuvant is a metal salt of acarboxylic acid, wherein the metal is chosen from the group consistingof lithium, magnesium, calcium, strontium, rubidium, barium, zinc,copper, tin, titanium, manganese, iron, vanadium, and aluminum.

This invention extends to a front plane laminate, double release sheet,inverted front plane laminate or electrophoretic display comprising anelectrophoretic medium of the present invention. The displays of thepresent invention may be used in any application in which prior artelectro-optic displays have been used. Thus, for example, the presentdisplays may be used in electronic book readers, portable computers,tablet computers, cellular telephones, smart cards, signs, watches,shelf labels and flash drives.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 of the accompanying drawings is a schematic cross-section throughan exemplary electrophoretic display.

FIGS. 2A and 2B are schematic cross-sections through an encapsulatedelectrophoretic display including magenta particles showing two opticalstates of the display under first and second voltages.

FIG. 3 shows white state contamination (WSc) as a function of washedthermogravimetric analysis (wTGA), where the wTGA values were modifiedby changing the level of functionalization and/or the amount of polymercoating.

FIG. 4 shows color saturation (CSc) as a function of washedthermogravimetric analysis (wTGA), where the wTGA values were modifiedby changing the level of functionalization and/or the amount of polymercoating.

FIG. 5 shows washed thermogravimetric analysis (wTGA) values as afunction of lauryl methacrylate (LMA) content for magenta pigmentsfunctionalized with glycidyl methacrylate (GMA) and4-vinylbenzylchloride (VBC).

FIG. 6 shows washed thermogravimetric analysis (wTGA) values as afunction of the amount of functionalization with glycidyl rnethacrylate(GMA) or 4-vinylbenzylchloride (VBC). The lauryl methacrylate contentwas constant at approximately 1.3 times the weight of the pigment.

DETAILED DESCRIPTION

The invention includes improved pigments for use in paints, coatings,filters, and electrophoretic displays. In general, a quinacridonepigment can be surface functionalized with a functional molecule, suchas glycidyl methacrylate, maleic anhydride, or 4-methacryloxyethyltrimellitic anhydride to create a functionalized pigment, whereupon thefunctional groups can be activated to bond hydrophobic polymers, therebycoating the pigment with the hydrophobic polymer. In some embodiments,the hydrophobic polymers will be methacrylates or acrylates, such aslauryl acrylate, lauryl methacrylate, 2-ethylhexyl acrylate,2-ethylhexyl methacrylate, hexyl acrylate, hexyl methacrylate, n-octylacrylate, n-octyl methacrylate, n-octadecyl acrylate, or n-octadecylmethacrylate. In some instances the quinacridone pigment will befunctionalized with glycidyl methacrylate, and then polymer coated withlauryl methacrylate, resulting in polymer coated pigments of Formula IV,below:

wherein R₂ is a hydrogen, C₁-C₃ alkyl group, or a halogen, and m and nare independently positive integers between 10 and 200. In otherembodiments, the quinacridone pigment will be functionalized with maleicanhydride, and then polymer coated with lauryl methacrylate, resultingin polymer coated pigments of Formula VIII, below:

wherein R₂ is a hydrogen, C₁-C₃ alkyl group, or a halogen, and m and nare independently positive integers between 10 and 200.

In some embodiments, the pigments will be formed from functionalizedpigments of Formula II:

wherein R₂ is a hydrogen, C₁-C₃ alkyl group, or a halogen, R₄ is —OH or—O[CH₂CH(CH₂OCOC(CH₃)CH₂)O]_(x)H, and x is an integer from 1 to 15. Forexample, the pigments may be of Formula IX:

The functionalized pigments of any of the Formulae above can be coatedwith hydrophobic polymers, i.e., as described herein.

Functionalized pigment precursors, e.g., quinacridone pigments, e.g.,quinacridone derivative pigments, typically exist as colored crystalsprior to being functionalized. Accordingly the quinacridone moleculesthat are functionalized using the methods of the invention are oftenlocated at the outer surface of the crystal. Accordingly, a varyingamount of functionalization may be achieved for individual quinacridonemolecules depending upon the location and orientation of thequinacridone molecule with respect to the larger crystal structure.Additionally, during the functionalization and processing, it is likelythat some amount of quinacridone will dissociate from the crystal,whereby the dissociated quinacridone may undergo completefunctionalization and widespread coupling to the provided hydrophobicpolymers. Details of quinacridone crystal structures can be found in theliterature, for example Panina et al., Journal of AppliedCrystallography (2007) p. 105-414, which is incorporated by reference inits entirety.

Displays of the present invention can reproduce the appearance of highquality color printing. Such high quality printing is typically effectedusing at least three colorants in a. subtractive primary color system,typically cyan/magenta/yellow (CMY) and optionally black. It is oftennot appreciated that a so-called three-color CMY printing system is inreality a four-color system, the fourth color being the white backgroundprovided by the substrate (paper or similar) surface to which colorantsare applied, and which performs the function of reflecting the lightfiltered by the subtractive colorants back to the viewer. Since there isno comparable background color in an essentially opaque electrophoreticmedium unless it is being used in shutter mode, a non-shutter-modeelectrophoretic medium should be capable of modulating four colors(white and three primary colors, the three primary colors typicallybeing cyan, magenta and yellow, or red, green and blue). Optionally ablack material may also be included, but it is possible to render blackby a combination of cyan, magenta and yellow colors.

Before describing in detail preferred electrophoretic media and displaysof the present invention, sonic general guidance will be given regardingmaterials for use in such media and displays, and preferred processesfor their preparation.

The materials and processes used in preparing the media and displays ofthe present invention are generally similar to those used in similarprior art media and displays. As described for example incommonly-assigned U.S. Pat. No. 6,822,782, a typical electrophoreticmedium comprises a fluid, a plurality of electrophoretic particlesdisposed in the fluid and capable of moving through the fluid (i.e.,translating, and not simply rotating) upon application of an electricfield to the fluid. The fluid also typically contains at least onecharge control agent (CCA), a charging adjuvant, and a polymericrheology modifier. These various components will now be describedseparately.

A: Fluid

The fluid contains the charged electrophoretic particles, which movethrough the fluid under the influence of an electric field. A preferredsuspending fluid has a low dielectric constant (about 2), high volumeresistivity (about 10¹⁵ Ohm·cm), low viscosity (less than 5 mPas), lowtoxicity and environmental impact, low water solubility (less than 10parts per million (ppm), if traditional aqueous methods of encapsulationare to be used; note however that this requirement may be relaxed fornon-encapsulated or certain microcell displays), a high boiling point(greater than about 90° C.), and a low refractive index (less than 1.5).The last requirement arises from the use of scattering (typically white)pigments of high refractive index, whose scattering efficiency dependsupon a mismatch in refractive index between the particles and the fluid.

Organic solvents such as saturated linear or branched hydrocarbons,silicone oils, halogenated organic solvents, and low molecular weighthalogen-containing polymers are some useful fluids. The fluid maycomprise a single component or may be a blend of more than one componentin order to tune its chemical and physical properties. Reactants orsolvents for the microencapsulation process (if used), such as oilsoluble monomers, can also be contained in the fluid.

Useful organic fluids include, but are not limited to, saturated orunsaturated hydrocarbons (such as, but are not limited to, dodecane,tetradecane, the aliphatic hydrocarbons in the ISOPAR® series (Exxon,Houston, Tex.), NORPAR® (Exxon, a series of normal paraffinic liquids),SHELL-SOL® (Shell, Houston, Tex.), and SOL-TROL® (Shell), naphtha, andother petroleum solvents; silicone oils (such as, but are not limitedto, octamethyl cyclosiloxane and higher molecular weight cyclicsiloxanes, poly(methyl phenyl siloxane), hexamethyldisiloxane, andpolydimethylsiloxane; vinyl ethers, such as cyclohexyl vinyl ether andDECANT® (International Flavors & Fragrances, Inc., New York, N.Y.);aromatic hydrocarbons, such as toluene; and halogenated materialsincluding, but not limited to, tetrafluorodibromoethylene,tetrachloroethylene, trifluorochloroethylene, 1,2,4-trichlorobenzene andcarbon tetrachloride and perfluoro- or partially-fluorinatedhydrocarbons.

It is advantageous in some electrophoretic media of the presentinvention for the fluid to contain an optically absorbing dye. This dyemust be soluble or dispersible in the fluid, but will generally beinsoluble in the other components of the microcapsule. There is muchflexibility in the choice of dye material. The dye can be a purecompound, or blends of dyes may be used to achieve a particular color,including black. The dyes can be fluorescent, which would produce adisplay in which the fluorescence properties depend on the position ofthe particles. The dyes can be photoactive, changing to another color orbecoming colorless upon irradiation with either visible or ultravioletlight, providing another means for obtaining an optical response. Dyescould also be polymerizable by, for example, thermal, photochemical orchemical diffusion processes, forming a solid absorbing polymer insidethe bounding shell.

Many dyes can be used in electrophoretic media. Important dye propertiesinclude light fastness, solubility or dispersibility in the fluid,color, and cost. The dyes are generally chosen from the classes of azo,azomethine, fluoran, anthraquinone, and triphenylmethane dyes and may bechemically modified so as to increase their solubility in the fluid andreduce their adsorption to the particle surfaces.

B: Electrophoretic Particles

The electrophoretic particles used in the media and displays of thepresent invention are preferably white, black, yellow, magenta, cyan,red, green, or blue in color, although other (spot) colors may also beused. There is much flexibility in the choice of such particles. Forpurposes of this invention, an electrophoretic particle is any particlethat is insoluble in the fluid and charged or capable of acquiring acharge (i.e., has or is capable of acquiring electrophoretic mobility).In some cases, this mobility may be zero or close to zero (i.e., theparticles will not move). The particles may be, for example,non-derivatized pigments or dyed (laked) pigments, or any othercomponent that is charged or capable of acquiring a charge. Typicalconsiderations for the electrophoretic particle are its opticalproperties, electrical properties, and surface chemistry. The particlesmay be organic or inorganic compounds, and they may either absorb lightor scatter light, i.e., the particles for use in the invention mayinclude scattering pigments, absorbing pigments and luminescentparticles. The particles may be retroreflective or they may beelectroluminescent, such as zinc sulfide particles, or they may bephotoluminescent.

The electrophoretic particle may have any shape, i.e., spherical,plate-like or acicular. A scattering particle typically has highrefractive index, high scattering coefficient, and low absorptioncoefficient and may be composed of an inorganic material such as rutile(titania), anatase (titania), barium sulfate, zirconium oxide, kaolin,or zinc oxide. Other particles are absorptive, such as carbon black orcolored organic or inorganic pigments such as are used in paints andinks. A reflective material can also be employed, such as a metallicparticle. Useful particle diameters may range from 10 nm up to about 10μm, although for light-scattering particles it is preferred that theparticle diameter not be smaller than about 200 nm.

Particularly preferred raw pigment particles of the present inventionare red pigments such as Pigment Red 112, Pigment Red 179, Pigment Red188 and Pigment Red 254 and magenta pigments such as Pigment Violet 19,Pigment Red 52:2 and Pigment Red 122. These pigments are based upon thequinacridone molecule, a version of which is shown below in Formula V:

wherein R is a hydrogen, C₁-C₃ alkyl group, or a halogen. Otherquinacridones, having structures not covered by Formula V, may also bepolymer coated using the techniques described herein. A pre-milledcombination of Pigment Violet 19 (CAS# 1047-16-1) and Pigment Red 122(CAS# 980-26-7) is commercially available from Clariant (Basel,Switzerland) as Ink Jet Magenta E 02 VP2621.

The surface treatments described herein may also be used to surface-coatother pigments where the chemistry of the pigment is conducive to themethods described herein. Raw pigments for use in the electrophoreticparticles include, but are not limited to, PbCrO₄, Cyan blue GT 55-3295(American Cyanamid Company, Wayne, N.J.), Cibacron Black BG (CibaCompany, Inc., Newport, Del.), Cibacron Turquoise Blue G (Ciba), CibalonBlack BGL (Ciba), Orasol Black BRG (Ciba), Orasol Black RBL (Ciba),Acetamine Black, CBS (E. I. du Pont de Nemours and Company, Inc.,Wilmington, Del., hereinafter abbreviated du Pont), Crocein Scarlet N Ex(du Pont) (27290), Fiber Black VF (du Pont) (30235), Luxol Fast Black L(du Pont) (Solv. Black 17), Nirosine Base No. 424 (du Pont) (50415 B),Oil Black BG (du Pont) (Solve Black 16), Rotalin Black RM (du Pont),Sevron Brilliant Red 3 B (du Pont); Basic Black DSC (Dye Specialties,Inc.), Hectolene Black (Dye Specialties, Inc.), Azosol Brilliant Blue B(GAF, Dyestuff and Chemical Division, Wayne, N.J.) (Solv. Blue 9),Azosol Brilliant Green BA (GAF) (Solv. Green 2), Azosol Fast BrilliantRed B (GAF), Azosol Fast Orange RA Conc. (GAF) (Solv. Orange 20), AzosolFast Yellow GRA Conc. (GAF) (13900 A), Basic Black KMPA (GAF), BenzofixBlack CW-CF (GAF) (35435), Cellitazol BNFV Ex Soluble CF (GAF) (Disp.Black 9), Celliton Fast Blue AF Ex Conc (GAF) (Disp. Blue 9), CyperBlack IA (GAF) (Basic Black 3), Diamine Black CAP Ex Cone (GAF) (30235),Diamond Black EAN Hi Con. CF (GAF) (15710), Diamond Black PBBA Ex (GAF)(16505); Direct Deep Black EA Ex CF (GAF) (30235), Hansa Yellow G (GAF)(11680); Indanthrene Black BBK Powd. (GAF) (59850), Indocarbon CLGSConc. CF (GAF) (53295), Katigen Deep Black NND Hi Conc. CF (GAF)(15711), Rapidogen Black 3 G (GAF) (Azoic Black 4); Sulphone CyanineBlack BA-CF (GAF) (26370), Zambezi Black VD Ex Conc. (GAF) (30015);Rubanox Red CP-1495 (The Sherwin-Williams Company, Cleveland, Ohio)(15630); Raven 11 (Columbian Carbon Company, Atlanta, Ga.), (carbonblack aggregates with a particle size of about 25 μm), Statex B-12(Columbian Carbon Co.) (a furnace black of 33 μm average particle size),Greens 223 and 425 (The Shepherd Color Company, Cincinnati, Ohio 45246);Blacks 1, 1G and 430 (Shepherd); Yellow 14 (Shepherd); Krolor YellowKO-788-D (Dominion Colour Corporation, North York, Ontario; KROLOR is aRegistered Trade Mark); Red Synthetic 930 and 944 (Alabama Pigments Co.,Green Pond, Ala. 35074), Krolor Oranges KO-786-1) and KO-906-D (DominionColour Corporation); Green GX (Bayer); Green 56 (Bayer); Light Blue ZR(Bayer); Fast Black 100 (Bayer); Bayferrox 130M (Bayer BAYFERROX is aRegistered Trade Mark); Black 444 (Shepherd); Light Blue 100 (Bayer);Light Blue 46 (Bayer); Yellow 6000 (First Color Co., Ltd., 1236-1,Jungwang-dung, Siheung-city Kyonggi-do, Korea 429-450), Blues 214 and385 (Shepherd); Violet 92 (Shepherd); and chrome green.

The electrophoretic particles may also include laked, or dyed, pigments.Laked pigments are particles that have a dye precipitated on them orwhich are stained. Lakes are metal salts of readily soluble anionicdyes. These are dyes of azo, triphenylmethane or anthraquinone structurecontaining one or more sulphonic or carboxylic acid groupings. They areusually precipitated by a calcium, barium or aluminum salt onto asubstrate. Typical examples are peacock blue lake (Cl Pigment Blue 24)and Persian orange (lake of Cl Acid Orange 7), Black M Toner (GAF) (amixture of carbon black and black dye precipitated on a lake).

It is preferred that pigments in the three subtractive primary colors(yellow, magenta and cyan) have high extinction coefficients andsufficiently small particle size as to be substantially non scatteringof incident light.

Additional pigment properties which may be relevant are particle sizedistribution and light-fastness. Composite particle (i.e., polymericparticles that incorporate smaller pigment particles or dyes) may beused in the present invention. Pigments may be surface-functionalized asdescribed below or may be used without functionalization.

It has long been known that the physical properties and surfacecharacteristics of electrophoretic panicles can be modified by adsorbingvarious materials on to the surfaces of the particles, or chemicallybonding various materials to these surfaces; see U.S. Pat. No.6,822,782, especially column 4, line 27 to column 5, line 32. This sameU.S. patent demonstrates that there is an optimum amount of polymerwhich should be deposited (too large a proportion of polymer in themodified particle causes an undesirable reduction in the electrophoreticmobility of the panicle) and that the structure of the polymer used toform the coating on the particle is important.

C: Charge Control Agents

The electrophoretic media of the present invention will typicallycontain a charge control agent (CCA), and may contain a charge director.These electrophoretic media components typically comprise low molecularweight surfactants, polymeric agents, or blends of one or morecomponents and serve to stabilize or otherwise modify the sign and/ormagnitude of the charge on the electrophoretic particles. The CCA istypically a molecule comprising ionic or other polar groupings,hereinafter referred to as head groups. At least one of the positive ornegative ionic head groups is preferably attached to a non-polar chain(typically a hydrocarbon chain) that is hereinafter referred to as atail group. It is thought that the CCA forms reverse micelles in theinternal phase and that it is a small population of charged reversemicelles that leads to electrical conductivity in the very non-polarfluids typically used as electrophoretic fluids.

Reverse micelles comprise a highly polar core (that typically containswater) that may vary in size from 1 nm to tens of nanometers (and mayhave spherical, cylindrical, or other geometry) surrounded by thenon-polar tail groups of the CCA molecule. Reverse micelles have beenextensively studied, especially in ternary mixtures such asoil/water/surfactant mixtures. An example is the iso-octane/water/AOTmixture described, for example, in Fayer et al., J. Chem. Phys., 131,14704 (2009). In electrophoretic media, three phases may typically bedistinguished: a solid particle having a surface, a highly polar phasethat is distributed in the form of extremely small droplets (reversemicelles), and a continuous phase that comprises the fluid. Both thecharged particles and the charged reverse micelles may move through thefluid upon application of an electric field, and thus there are twoparallel pathways for electrical conduction through the fluid (whichtypically has a vanishingly small electrical conductivity itself).

The polar core of the CCA is thought to affect the charge on surfaces byadsorption onto the surfaces. In an electrophoretic display, suchadsorption may be onto the surfaces of the electrophoretic particles orthe interior walls of a microcapsule (or other solid phase, such as thewalls of a microcell) to foam structures similar to reverse micelles,these structures hereinafter being referred to as hemi-micelles. Whenone ion of an ion pair is attached more strongly to the surface than theother (for example, by covalent bonding), ion exchange betweenhemi-micelles and unbound reverse micelles can lead to charge separationin which the more strongly bound ion remains associated with theparticle and the less strongly bound ion becomes incorporated into thecore of a free reverse micelle.

It is also possible that the ionic materials forming the head group ofthe CCA may induce ion-pair formation at the particle (or other)surface. Thus the CCA may perform two basic functions: charge-generationat the surface and charge-separation from the surface. Thecharge-generation may result from an acid-base or an ion-exchangereaction between some moiety present in the CCA molecule or otherwiseincorporated into the reverse micelle core or fluid, and the particlesurface. Thus, useful CCA materials are those which are capable ofparticipating in such a reaction, or any other charging reaction asknown in the art.

Non-limiting classes of charge control agents which are useful in themedia of the present invention include organic sulfates or sulfonates,metal soaps, block or comb copolymers, organic amides, organiczwitterions, and organic phosphates and phosphonates. Useful organicsulfates and sulfonates include, but are not limited to, sodiumbis(2-ethylhexyl) sulfosuccinate, calcium dodecylbenzenesulfonate,calcium petroleum sulfonate, neutral or basic barium dinonylnaphthalenesulfonate, neutral or basic calcium dinonylnaphthalene sulfonate,dodecylbenzenesulfonic acid sodium salt, and ammonium lauryl sulfate.Useful metal soaps include, but are not limited to, basic or neutralbarium petronate, calcium petronate, cobalt, calcium, copper, manganese,magnesium, nickel, zinc, aluminum and iron salts of carboxylic acidssuch as naphthenic, octanoic, oleic, palmitic, stearic, and myristicacids and the like. Useful block or comb copolymers include, but are notlimited to, AB diblock copolymers of (A) polymers of2-(N,N-dimethylamino)ethyl methacrylate quaternized with methylp-toluenesulfonate and (B) poly(2-ethylhexyl methacrylate), and combgraft copolymers with oil soluble tails of poly(12-hydroxystearic acid)and having a molecular weight of about 1800, pendant on an oil-solubleanchor group of poly(methyl methacrylate-methacrylic acid). Usefulorganic amides/amines include, but are not limited to, polyisobutylenesuccinimides such as OLOA 371 or 1200 (available from Chevron OroniteCompany LLC, Houston, Tex.), or SOLSPERSE® 17000 (available fromLubrizol, Wickliffe, Ohio), and N-vinylpyrrolidone polymers. Usefulorganic zwitterions include, but are not limited to, lecithin. Usefulorganic phosphates and phosphonates include, but are not limited to, thesodium salts of phosphated mono- and di-glycerides with saturated andunsaturated acid substituents. Useful tail groups for CCA includepolymers of olefins such as polyisobutylene) of molecular weight in therange of 200-10,000. The head groups may be sulfonic, phosphoric orcarboxylic acids or amides, or alternatively amino groups such asprimary, secondary, tertiary or quaternary ammonium groups.

Charge adjuvants used in the media of the present invention may bias thecharge on electrophoretic particle surfaces, as described in more detailbelow. Such charge adjuvants may be Bronsted or Lewis acids or bases.

Particle dispersion stabilizers may be added to prevent particleflocculation or attachment to the capsule or other walls or surfaces.For the typical high resistivity liquids used as fluids inelectrophoretic displays, non-aqueous surfactants may be used. Theseinclude, but are not limited to, glycol ethers, acetylenic glycols,alkanolamides, sorbitol derivatives, alkyl amines, quaternary amines,imidazolines, dialkyl oxides, and sulfosuccinates.

D: Polymeric Additives

As described in U.S. Pat. No. 7,170,670, the bistability ofelectrophoretic media can be improved by including in the fluid apolymer having a number average molecular weight in excess of about20,000, this polymer being essentially non-absorbing on theelectrophoretic particles; polyisobutylene) is a preferred polymer forthis purpose.

Also, as described in for example, U.S. Pat. No. 6,693,620, a particlewith immobilized charge on its surface sets up an electrical doublelayer of opposite charge in a surrounding fluid. Ionic head groups ofthe CCA may be ion-paired with charged groups on the electrophoreticparticle surface, forming a layer of immobilized or partiallyimmobilized charged species. Outside this layer is a diffuse layercomprising charged (reverse) micelles comprising CCA molecules in thefluid. In conventional DC electrophoresis an applied electric fieldexerts a force on the fixed surface charges and an opposite force on themobile counter-charges, such that slippage occurs within the diffuselayer and the particle moves relative to the fluid. The electricpotential at the slip plane is known as the zeta potential.

FIG. 1 of the accompanying drawings is a schematic cross-section throughan electrophoretic display (generally designated 100) of the presentinvention comprising an encapsulated electrophoretic medium; such adisplay, and methods for its manufacture are described in U.S. Pat. No.6,982,178. The display 100 comprises a light-transmissive substrate 102,typically a transparent plastic film, such as a sheet of polyethyleneterephthalate) (PET) about 25 to 200 μm in thickness. Although not shownin FIG. 1, the substrate 102 (the upper surface of which, as illustratedin FIG. 1, forms the viewing surface of the display) may comprise one ormore additional layers, for example a protective layer to absorbultra-violet radiation, barrier layers to prevent ingress of oxygen ormoisture into the display, and anti-reflection coatings to improve theoptical properties of the display.

The substrate 102 carries a thin, light-transmissive,electrically-conductive layer 104 that acts as the front electrode ofthe display. Layer 104 may comprise a continuous coating ofelectrically-conductive material with minimal intrinsic absorption ofelectromagnetic radiation in the visible spectral range such as indiumtin oxide (ITO), poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)(PEDOT:PSS), graphene and the like, or may be a discontinuous layer of amaterial such as silver (in the form of, for example, nanowires orprinted grids) or carbon (for example in nanotube form) that absorb orreflect visible light but are present at such a surface coverage thatthe layer as a whole is effectively transparent.

A layer (generically designated 108) of an electrophoretic medium is inelectrical contact with the conductive layer 104 through an optionalpolymeric layer or layers 106, as described in more detail below. Theelectrophoretic medium 108 is shown as an encapsulated electrophoreticmedium comprising a plurality of microcapsules. The microcapsules may beretained within a polymeric binder. Upon application of an electricalfield across the layer 108, negatively-charged particles therein movetowards the positive electrode and positively-charged particles movetowards the negative electrode, so that the layer 108 appears, to anobserver viewing the display through the substrate 102, to change color.

Although the display 100 is illustrated as having an encapsulatedelectrophoretic layer 108, this is not an essential feature of thepresent invention. Layer 108 may be encapsulated or comprise sealed orunsealed micro-cells or micro-cups, or may be non-encapsulated. When thelayer is non-encapsulated, the electrophoretic internal phase (theelectrophoretic particles and fluid) may be located between two planarelectrodes, at least one of which is light-transmissive. The spacingbetween the electrodes may be controlled by the use of spacers, whichmay have the form of ribs or beads. Alternatively, the spacing may becontrolled by the use of microcapsules containing the internal phase;the internal phase may be located within and outside the capsules. It isnot necessary that the internal phase inside and outside themicrocapsules be identical, although in certain circumstances this maybe preferred. For example, if capsules containing the same internalphase as that outside the capsules are used as spacers it may be thatthe presence of the spacers is less easily discernible by a viewer ofthe display (since the internal and external internal phases wouldswitch to at least substantially the same color).

As described in U.S. Pat. Nos. 6,982,178 and 7,012,735, the display 100further comprises a layer 110 of lamination adhesive covering theelectrophoretic layer 108. The lamination adhesive makes possible theconstruction of an electro-optic display by combining two subassemblies,namely a backplane 118 that comprises an array of pixel electrodes 112and an appropriate arrangement of conductors to connect the pixelelectrodes to drive circuitry, and a front plane 116 that comprises thesubstrate 102 bearing the transparent electrode 104, the electrophoreticlayer 108, the lamination adhesive 110 and optional additionalcomponents such as polymeric layer or layers 106. To form the finaldisplay, the front plane 116 is laminated to the backplane 118 by meansof lamination adhesive 110. The lamination adhesive may be curedthermally or by actinic radiation (for example, by UV curing) or may beuncured.

Since the lamination adhesive 110 is in the electrical path from thebackplane electrodes 112 to the front electrode 104, its electricalproperties must be carefully tailored. As described in U.S. Pat. No.7,012,735 the lamination adhesive may comprise, in addition to apolymeric material, an ionic dopant that may be an additive selectedfrom a salt, a polyelectrolyte, a polymer electrolyte, a solidelectrolyte, a conductive metal powder, a ferrofluid, a non-reactivesolvent, a conductive organic compound, and combinations thereof. Thevolume resistivities of encapsulated electrophoretic media of thepresent invention are typically around 10¹⁰ Ohm·cm, and theresistivities of other electro-optic media are usually of the same orderof magnitude. Accordingly, the volume resistivity of the laminationadhesive is normally around 10⁸ to 10¹² Ohm·cm at the operatingtemperature of the display; which is typically around 20° C.

Polymeric layer 106 may be a lamination adhesive layer with similarproperties to those of lamination adhesive layer 110 (see for exampleU.S. Pat. No. 7,839,564), except that, since polymeric layer 106 isadjacent to the non-pixelated, light-transmissive common electrode 104,its electrical conductivity may be higher than that of laminationadhesive layer 110, which is adjacent to the pixelated back planeelectrodes 112 and cannot be so conductive as to lead to significantcurrents flowing from one backplane electrode to its neighbors when theyare held at different potentials during switching of the display. Whenpolymeric layer 106 is a lamination adhesive it may be used to affixelectrophoretic layer 108 to front electrode 104 during manufacture ofthe front plane as described in detail in the aforementioned U.S. Pat.No. 6,982,178.

As mentioned above, the present invention provides an electrophoreticmedium comprising a fluid and at least a first species of particlesdisposed in the fluid, the first species of particles being such thatwhen a first electric field is applied to the medium for a first period,thereby applying a first addressing impulse to the medium, the firstspecies of particles move in one direction relative to the electricfield, but when a second electric field, having the same polarity as thefirst electric field, is applied to the medium for a second period,thereby applying a second addressing impulse larger than the firstaddressing impulse to the medium, the first species of particles move inthe opposed direction relative to the electric field. For the purpose ofproviding a better understanding of the present invention, the followinghypothesis as to how the pigment particles might move in a firstdirection with a first addressing impulse (i.e., behaving as though theparticles bore a negative charge) and in a second direction with asecond, higher addressing impulse (i.e., behaving as though theparticles bore a positive charge) is provided, but the invention is inno way limited by this hypothesis.

Various embodiments of electrophoretic media and displays of the presentinvention, and their use to form colored images, will now be describedin more detail. In these embodiments the following general switchingmechanisms are utilized:

-   -   (A) Conventional electrophoretic motion, in which particles with        associated charge (either surface-bound or adsorbed) move in an        electric field;    -   (B) Conventional racing particles, wherein particles of higher        zeta potential move faster than particles of lower zeta        potential (as described, for example, in U.S. Pat. No. 8,441,714        and earlier patents cited therein)    -   (C) Coulombic aggregation between particles of opposite sign,        such that the aggregate moves in an electric field according to        its net charge in the absence of an electrochemical (or        displacement) current, but wherein the aggregate is separated by        modulation of charge on at least one of the particles by the        electrochemical (or displacement) current;    -   (D) Reversal of the direction of motion of at least one species        of particles as a result of electrochemical (or displacement)        current.

The waveforms used to drive displays of the present invention maymodulate the electrical impulse provided to the display using any one ormore of at least four different methods:

-   -   (i) Pulse width modulation, in which the duration of a pulse of        a particular voltage is changed;    -   (ii) Duty cycle modulation, in which a sequence of pulses is        provided whose duty cycle is changed according to the impulse        desired;    -   (iii) Voltage modulation, in which the voltage supplied is        changed according to the impulse required; and    -   (iv) A DC voltage offset applied to an AC waveform (which itself        has net zero impulse)

Which of these methods is used depends upon the intended application andthe exact form of display used. As noted above, herein the term impulseis used to denote the integral of the applied voltage with respect totime during the period in which a medium or display is addressed. Alsoas noted above, a certain electrochemical or displacement current isrequired for the change in direction of a (typically negatively-charged)species of particle or the disaggregation of Coulombic aggregates, andthus when a high impulse is to applied to a medium of display, theaddressing voltage must be sufficient to provide such a current. Lowerimpulses may be provided by lower addressing voltages, or by reductionin the addressing time at the same higher voltage. As noted above, thereis a polarization phase during which electrochemical currents are not attheir maximum value, and during this polarization phase the particlesmove according to their native charge (i.e., the charge they bear beforeany addressing voltage is applied to the medium or display. Thus,low-impulse addressing at high voltage is ideally for a duration such asto polarize the electrophoretic medium but not lead to high steady-statecurrent flow.

FIGS. 2A and 2B are schematic cross-sections showing various possiblestates of single microcapsule 800 (a sealed or unsealed microcell, orother similar enclosure may alternatively be used), containing a fluid806 dyed with a yellow dye (uncharged yellow particles may besubstituted for the yellow dye. Disposed in the fluid 806 arepositively-charged light-transmissive magenta particles 802 andnegatively-charged white particles 804. On the upper side ofmicrocapsule 800, as illustrated in FIGS. 2A and 2B, is a substantiallytransparent front electrode 810, the upper surface of which (asillustrated) forms the viewing surface of the display, while on theopposed side of the microcapsule 800 is a rear or pixel electrode 812.In FIGS. 2A and 2B, it will be assumed that the front electrode 810remains at ground potential (although this is not an essential featureof the present invention, and variation of the potential of thiselectrode may be desirable in some instances, for example to providehigher electric fields), and that the electric field across microcapsule800 is controlled by changing the voltage of the rear electrode 812.

FIGS. 2A and 2B illustrate separation of charged color particles (802and 804) within the microcapsule 800 when a voltage is applied across anelectrophoretic medium, thereby causing particles 802 and 804 to undergoelectrophoretic motion. As shown in FIG. 2A, when the rear electrode 812is at a positive voltage, the white particles 804 move towards the rearelectrode 812, while the magenta particles 802 lie adjacent the frontelectrode 810. In this configuration, the microcapsule 800 displays ared color caused by the combination of the magenta particles and theyellow dye viewed against the white background provided by the whiteparticles. (View is assumed to be above, but in the approximate plane ofthe figure in FIGS. 2A and 2B,) As shown in FIG. 2B, when the rearelectrode 812 is at a negative voltage, the white particles 804 moveadjacent the front electrode 810, and the microcapsule 800 displays awhite color (both the yellow fluid 806 and the magenta particles 802 aremasked by the white particles 804).

Obviously, other combinations of colored particles and dyes can besubstituted for the white and magenta particles, and yellow dye, used inFIGS. 2A and 2B. Especially preferred embodiments of the presentinvention are those in which one dye or particle has one of the additiveprimary colors, and another is of the complementary subtractive primarycolor. Thus, for example, the dye might be cyan and the two particleswhite and red. Other combinations of particles, including multipleparticle groups having the same charge polarity, but different chargemagnitudes, are also possible. Additionally, green/magenta andblue/yellow combinations of dye and particle may be used, together witha white particle. The fluid 806 may optionally be colorless.

In one preferred embodiment of the present invention, the first (white)particle is a silanol-functionalized scattering material such astitanium dioxide to which a polymeric material has been attached; thesecond particle is a positively charged magenta material such asquinacridone pigment that has been coated as described below.

An electrophoretic display may additionally include a dye. The dye insome embodiments is a hydrocarbon (ISOPAR® E)-soluble material that maybe an azo dye such as Sudan I or Sudan II or derivatives thereof. Otherhydrocarbon-soluble dyes such as azomethine (yellow and cyan are readilyavailable) or other materials that are well-known in the art may alsobe.

The following Examples are now given, though by way of illustrationonly, to shows details of particularly preferred materials, processes,conditions and techniques used to prepare the media and electrophoreticdisplays of the present invention.

EXAMPLE 1 GMA Functionalized Pigments

This Example illustrates the preparation of a magenta polymer coatedpigment and the incorporation of the pigment into a test display of thetype illustrated in the accompanying drawings.

Part A: Preparation of a Magenta Pigment Dispersion

Ink Jet Magenta E 02 VP2621, available from Clariant, Basel,Switzerland, was dispersed in toluene. The resultant dispersion wastransferred to a 500 mL round-bottomed flask and the flask degassed withnitrogen. The reaction mixture was then brought to 45° C., and, upontemperature equilibration, glycidyl methacrylate (GMA) monomer was addedand the reaction was allowed to stir at 45° C. for four hours. Theresulting reaction mixture was allowed to cool to room temperature andthen poured into a 1 L plastic centrifuge bottle, diluted with tolueneand centrifuged at 3500 RPM for 20 minutes. The centrifuge cake waswashed twice with toluene, each time the mixture was centrifuged at 3500RPM for 20 minutes. After the final wash, the supernatant was decantedand the resultant pigment was dried in a 70° C. vacuum oven overnight,then ground with a mortar and pestle. This procedure produces magentapigment functionalized with an acrylate group to which a polymeric chaincould be attached.

The dried functionalized pigment was then dispersed in toluene andlauryl methacrylate (LMA) monomer with ball milling with Zirconoxgrinding media and a roll mill, and the resultant dispersion transferredto a jacketed 500 mL rector equipped with an overhead stirrer andbrought to 65° C. via a circulating water bath. The system was purgedwith nitrogen for at least one hour, and then a solution of AWN(2,2′-azobis(2-methylpropionitrile)) in toluene was metered into thereaction. The reaction mixture was stirred vigorously at 65° C. for 16hours, then poured into a 1 L plastic centrifuge bottle, diluted withtoluene and centrifuged at 4500 RPM for 30 minutes. The centrifuge cakewas washed once with toluene and the mixture was again centrifuged at4500 RPM for 30 minutes. The supernatant was decanted and the resultantpigment was dried in a 70° C. vacuum oven overnight. This procedureproduces a magenta pigment with a covalently bound polymeric shell witha typical molecular weight of 35-120 kDA. See Formula IL The polymerizedpigment was then ground with a mortar and pestle, and dispersed inISOPAR® E to form a 20 weight % dispersion, which was sonicated androlled on a roll mill for 24 hours. The resultant dispersion wasfiltered through fabric mesh to remove any large particles, a sampleremoved and its solids content measured.

Part B: Preparation of Internal Phase

The magenta pigment dispersion prepared in Part A above (13.92 g of a14% w/w dispersion in ISOPAR® E) was combined with 83.07 g of a 60% w/wISOPAR® E dispersion of titanium dioxide (polymer coated as described inthe aforementioned U.S. Pat. No. 7,002,728), 7.76 g of a 20% w/wsolution of SOLSPERSE® 17000 in ISOPAR® E, a 15% w/w solution ofpolyisobutylene) of molecular weight 1270 kDa in ISOPAR® E (thispolyisobutylene) acts as an image stabilizer; see U.S. Pat. No.7,170,670), and 5.82 g of ISOPAR® E. The resultant mixture was dispersedovernight on a mechanical roller to produce an internal phase ready forencapsulation and having a conductivity of 304.7 pS/cm.

Part C: Microencapsulation

The internal phase prepared in Part B was encapsulated following theprocedure described in U.S. Pat. No. 7,002,728. The resultantencapsulated material was isolated by sedimentation, washed withdeionized water, and size-separated by sieving. Capsule size analysisusing a Coulter Multisizer showed that the resulting capsules had a meansize of 40 pm and more than 85 percent of the total capsule volume wasin capsules having the desired size of between 20 and 60 μm.

Part D: Preparation of Display

The sieved capsules produced in Part C above were adjusted to pH 9 withammonium hydroxide solution and excess water removed. The capsules werethen concentrated and the supernatant liquid discarded. The concentratedcapsules were mixed with an aqueous polyurethane binder (prepared in amanner similar to that described in U.S. Patent Application PublicationNo. 2005/0124751) at a ratio of 1 part by weight binder to 15 parts byweight of capsules following which Triton X-100 surfactant andhydroxypropylmethylcellulose were added and mixed thoroughly to providea slurry.

The capsule slurry thus prepared was coated onto the indium tin oxide(ITO) coated surface of a polyethylene terephthalate) (PET)/ITO film of125 μm thickness using a bar coater, and the coated film dried at 60° C.Separately, a layer of polyurethane adhesive doped withtetraethylarnmoniurn hexafluorophosphate as a conductive dopant wascoated onto a release sheet, and the resultant PET film/adhesivesub-assembly was laminated on top of the coated capsules as described inthe above-mentioned U.S. Pat. No. 7,002,728. The release sheet wasremoved and the resultant multilayer structure was laminated onto agraphite rear electrode to produce an experimental single-pixel displaycomprising, in order from its viewing surface, the PET film, a layer ofITO, a capsule layer, a lamination adhesive layer, and the graphite rearelectrode.

Part E: Electro-Optic Tests

The resulting displays were switched using a square-wave AC waveformapplied to the graphite rear electrode (while the front ITO electrodewas grounded) of ±30V and 50 Hz that was offset from zero as specifiedbelow. The DC offset for red/white switching was ±10V. In this case, thewhite and magenta pigments move though the non-polar fluid. The redstate results from viewing of the magenta (green-absorbing) pigmentagainst the white background. The DC offset for magenta/white switchingwas ±60V The white color was obtained as the white pigment moved awayfrom the viewing side of the display towards the negatively-charged rearelectrode, as described above with reference to FIGS. 2A and 2B.

Magenta pigments functionalized with glycidyl methacrylate (GMA) andcoated with lauryl methacrylate were compared to state-of-the artmagenta pigments. The state-of-the-art pigments were produced asdescribed in U.S. Patent Application No. 2014/0340430, which isincorporated by reference in its entirety. The state-of-the-art magentapigments were produced in a manner similar to that described above inPart A, however the magenta pigment was functionalized with4-vinylbenzylchloride (VBA) prior to coating with lauryl methacrylate(LMA). The amount of functionalized monomer and the amount of surfacecoating polymer were also varied to evaluate how these features alteredthe overall performance of the pigment in an electrophoretic display.

Experience with colored electrophoretic systems using hydrophobic fluidssuggests that better switching and color states are achieved withpigments having high organics content, typically representing a pigmentwith a dense polymer shell around the pigment. The quality of thepolymer coating can be evaluated with a number of analytical techniquesand/or imaging. Nonetheless, it is very straightforward to evaluatepigments for organics content using thermogravimetric analysis (TGA),(FIGS. 3 and 4 refer to “washed” TGA because the pigments were washed intetrahydrofuran to assure that any loose polymer was removed from thepigment.) As shown in FIGS. 3 and 4, the overall performance of acolored/white pigment system in an electrophoretic display roughlytracks TGA. This is not strictly the case, however; white statecontamination (WS c) and color saturation (CS c) do vary somewhat formagenta pigments with the same polymer coating weight but differentfunctional elements (GMA vs. VBC). Compare, for example, the points forGMA and VBC with a wTGA of about 7%, where the white state contaminationof the GMA pigment particles is clearly better.

A series of functionalized and polymer coated pigments were prepared asshown in Table 1. The pigments were analyzed for organic content usingthermogravimetric analysis, as discussed above.

FIG. 5 shows that pigments functionalized with GMA, in general, have ahigher wTGA than similar pigments functionalized with VBC, when thelevel of LMA added during the coating step is identical. This result islikely due to improved grafting of LMA polymer with theGMA-functionalized pigments. As discussed above, the improved wTGAlevels for GMA functionalized pigments will correlated with improvedfunctionality in electrophoretic displays because of the higher organiccontent. It is also notable that increasing stoichiometric quantities ofGMA result in higher organic content, as evidenced by FIG. 6 and theblue starred point in FIG. 5 corresponding to 2xGMA_1.75xLMA in Table 1.An additional benefit of using GMA instead of VBC is that GMA has ahigher reactivity toward quinacridone pigments than VBC, thus thefunctionalization reaction can be completed in 2-4 hr at 45° C. versusovernight at 42° C. for VBC. See U.S. Patent Application No.2014/0340430.

EXAMPLE 2 Maleic Anhydride Functionalized Pigments

Part A: Preparation of a Magenta Pigment Dispersion

Ink Jet Magenta E 02 VP2621, available from Clariant, Basel,Switzerland, was dispersed in toluene. The resultant dispersion wastransferred to a 500 mL round-bottomed flask and the flask degassed withnitrogen. The reaction mixture was then brought to 45° C., and, upontemperature equilibration, maleic anhydride was added and the reactionwas allowed to stir at 65° C. for sixteen hours. The resulting reactionmixture was allowed to cool to room temperature and then poured into a 1L plastic centrifuge bottle, diluted with toluene and centrifuged at3500 RPM for 20 minutes. The centrifuge cake was washed twice withtoluene, each time the mixture was centrifuged at 3500 RPM for 20minutes. After the final wash, the supernatant was decanted and theresultant pigment was dried in a 70° C. vacuum oven overnight, thenground with a mortar and pestle. This procedure produces magenta pigmentfunctionalized with an acrylate group to which a polymeric chain couldbe attached. See Formula IV.

The dried functionalized pigment thus was dispersed in toluene andlaurel methacrylate (LMA) monomer with ball milling with Zirconoxgrinding media and a roll mill, and the resultant dispersion transferredto a jacketed 500 mL rector equipped with an overhead stirrer andbrought to 65° C. via a circulating water bath. The system was purgedwith nitrogen for at least one hour, and then a solution of AIBN(2,2′-azobis(2-methylpropionitrile)) in toluene was metered into thereaction. The reaction mixture was stirred vigorously at 65° C. for 16hours, then poured into a 1 L plastic centrifuge bottle, diluted withtoluene and centrifuged at 4500 RPM for 30 minutes. The centrifuge cakewas washed once with toluene and the mixture was again centrifuged at4500 RPM for 30 minutes. The supernatant was decanted and the resultantpigment was dried in a 70° C. vacuum oven overnight. This procedureproduces a magenta pigment with a covalently bound polymeric shell witha typical molecular weight of 35-120 kDA. See Formula V. The polymerizedpigment was then ground with a mortar and pestle, and dispersed inISOPAR® E to form a 20 weight % dispersion, which was sonicated androlled on a roll mill for 24 hours. The resultant dispersion wasfiltered through fabric mesh to remove any large particles, a sampleremoved and its solids content measured.

EXAMPLE 3 (Prophetic)—4-Methacryloxyethyl Trimellitic AnhydrideFunctionalized Pigments

Part A: Preparation of a Magenta Pigment Dispersion

Ink jet Magenta E 02 VP2621, available from Clariant, Basel,Switzerland, will be dispersed in toluene. The resultant dispersion willbe transferred to a 500 mL round-bottomed flask and the flask degassedwith nitrogen. The reaction mixture will then be brought to 45° C., and,upon temperature equilibration, 4-methacryloxyethyl trimelliticanhydride will be added and the reaction was allowed to stir at 65° C.for sixteen hours. The resulting reaction mixture will be allowed tocool to room temperature and then poured into a 1 L plastic centrifugebottle, diluted with toluene and centrifuged at 3500 RPM for 20 minutes.The centrifuge cake will be washed twice with toluene, each time themixture will be centrifuged at 3500 RPM for 20 minutes. After the finalwash, the supernatant will be decanted and the resultant pigment will bedried in a 70° C. vacuum oven overnight, then ground with a mortar andpestle. This procedure will produce magenta pigment functionalized withan acrylate group to which a polymeric chain could be attached.

The dried functionalized pigment will be dispersed in toluene and laurylmethacrylate (LMA) monomer with ball milling with Zirconox grindingmedia and a roll mill, and the resultant dispersion will be transferredto a jacketed 500 mL rector equipped with an overhead stirrer andbrought to 65° C. via a circulating water bath. The system will bepurged with nitrogen for at least one hour, and then a solution of AIBN(2,2′-azobis(2-methylpropionitrile)) in toluene will be metered into thereaction. The reaction mixture will be stirred vigorously at 65° C. for16 hours, then poured into a 1 L plastic centrifuge bottle, diluted withtoluene and centrifuged at 4500 RPM for 30 minutes. The centrifuge cakewill be washed once with toluene and the mixture will be againcentrifuged at 4500 RPM for 30 minutes. The supernatant will be decantedand the resultant pigment will be dried in a 70° C. vacuum ovenovernight. This procedure will produce a magenta pigment with acovalently bound polymeric shell with a typical molecular weight of35-120 kDA. The polymerized pigment will be ground with a mortar andpestle, and dispersed in ISOPAR® E to form a 20 weight % dispersion,which will be sonicated and rolled on a roll mill for 24 hours. Theresultant dispersion will be filtered through fabric mesh to remove anylarge particles, a sample will be removed and its solids content will bemeasured.

It will be apparent t© those skilled in the art that numerous changesand modifications can be made in the specific embodiments of theinvention described above without departing from the scope of theinvention. Accordingly, the whole of the foregoing description is to beinterpreted in an illustrative and not in a limitative sense.

1. A pigment comprising Formula VI:

wherein R₁ is a hydrogen, a C₁-C₃ alkyl group, a halogen, a hydroxyl, or—COCR₃CR₄COOH; R₂ is a hydrogen, C₁-C₃ alkyl group, or a halogen; R₃ ishydrogen or a hydrophobic polymer having a molecular weight between 5 kDand 100 kD; and R₄ is hydrogen or a hydrophobic polymer having amolecular weight between 5 kD and 100 kD.
 2. The pigment of claim 1,wherein the hydrophobic polymer is a polymer comprising lauryl acrylate,lauryl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate,hexyl acrylate, hexyl methacrylate, n-octyl acrylate, n-octylmethacrylate, n-octadecyl acrylate, n-octadecyl methacrylate, or acombination thereof.
 3. The pigment of claim 1, comprising Formula VII:

wherein m and n are independently integers between 10 and
 200. 4. Thepigment of claim 1, comprising Formula VIII:

wherein m and n are independently integers between 10 and
 200. 5. Thepigment of claim 1, wherein R₂ is —H or R₂ is —CH₃.
 6. Anelectrophoretic a medium comprising a pigment of claim
 1. 7. Anelecto-optic display comprising a pigment of claim
 1. 8. A front planelaminate comprising a pigment of claim 1.